1. Field of the Invention
The present invention relates to a laser beam scanning apparatus and, more specifically, to a laser beam scanning apparatus in which a laser beam from a laser beam generating source is deflected by a deflector, whereby a recording medium is scanned and an image is formed thereon.
2. Description of the Prior Art
Generally, optical apparatuses for printers employing a laser beam carrying image information such as shown in Japanese Patent Laying-Open Gazette No. 3114/1986 and in Japanese Patent Laying-Open Gazette No. 93026/1983 are well known. Compared with an electrophotographic copying machine (plane paper copier) utilizing projected images of originals by the visible light, higher image quality is required for a printer. Therefore, the optical apparatus is a critical portion dominating the image quality, and various portions thereof should be adjusted when assembled. Such adjustment comprises adjustment of the distance between the semiconductor laser and a collimator lens, co-axializing of the two, adjustment of the angular position of the fore and aft position of a cylindrical lens, adjustment of the fore and aft position of an image reflecting mirror, and so on. The fore and aft position of the image reflecting mirror is adjusted by moving the mirror back and forth in the direction of the light axis to change the distance between the point of deflection by the deflector to the surface of the recording medium, thereby adjusting the magnification of main scanning.
When a polygon mirror, a hologram scan, a galvano mirror and the like having a plurality of equally divided planes of deflection is used as the deflector in such optical apparatus, a sensor (hereinafter referred to as SOS sensor) is provided for detecting the start point of image formation in order to compensate for the division error between each of the planes of deflection. The SOS sensor should be preferably set at an equivalent position to the recording medium. The elements constituting the optical apparatus should preferably be provided in a unit, so as to facilitate the positioning of the apparatus to the body of the printer, to reduce vibration and to facilitate the care of the apparatus such as exchange of parts.
However, as the distance between the deflector and the recording medium becomes longer as the width of main scanning becomes larger, there arises various problems in setting the SOS sensor at a position equivalent to the recording medium. More specifically, if the SOS sensor is arranged out of the optical unit, the advantages of the unit are missed. If the SOS sensor is provided in the unit so as to maintain the advantages, the optical unit itself becomes larger. Therefore, it becomes difficult to contain the unit in the printer body due to the space requirement, which may cause enlargement of the printer body.
In view of the foregoing, an apparatus is proposed in which a number of mirrors used exclusively for the sensor are arranged so as to reflect the light path at a number of points. However, this requires a larger number of parts, and requires much space, thereby causing the enlargement of the apparatus.